However, my wall slants inwards 8 degrees ("inwards" meaning that the bottom of the wall is further out than the top). How should I handle this so that the mount is ultimately vertical? One thought I had was to have a 2x4 cut to the right dimensions with one side having an 8 degree angle to match the wall. I'd then place this on the bottom of where the mount will go, ultimately leaving me with studs > 2x4 > plywood > mount on the bottom, and studs > plywood > mount on the top.

is the wall slant intentional by design or due to weird house settling? If the TV is high on the wall, I'd say the 8% slant could actually be useful.
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DA01Jan 8 '13 at 4:00

Any reason you can't just wedge in shims to get the right angle?
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SteveJan 8 '13 at 4:04

Really?? 8° is significant: if you have an 8' ceiling (and my math is correct) that means the bottom is out 13.5" from the top..
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gregmacJan 8 '13 at 5:11

@gregmac - Your math is correct/ (I got 13.49" over a 8' high wall). It could be that the wall in question here is one inside of a building with a steep sided roof. A room above the roof line in such building could show a wall like this. Consider the style of roofs on old style barns that were originally designed for hay storage in the upper portion under the tall roof.
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Michael KarasJan 8 '13 at 6:35

Some TV mounts extend out from the wall on the bottom (to give you room to attach connections) and can be locked in this position with a bolt. Check your mount before you customize the plywood too much.
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BMitch♦Jan 8 '13 at 17:04

3 Answers
3

Your approach sounds perfect to me. 3 tips:
1. Err on the side of thicker plywood. 3/4" sanded pine is what I'd be thinking. 3/4 MDF would probably do fine as well.
2. You can use pretty much any CAD program to tell you the correct angles/dimensions for the upper and lower wedge-shaped 2x4's.
3. Upper wedge? Yes, I'd cut a thin wedge of 2x4 to put at the top as well, so that the fasteners aren't trying to bend the top edge of the plywood as you draw them up tight.